Assisted GPS (aGPS)

For a while now, a rumor has been going through the Internet about aGPS or Asissted GPS or in full Asissted Global Positioning System.

The rumor is that aGPS is worse then GPS, or even, is no GPS at all.

So when a spec sheet of a new device comes along and it lists aGPS, people freak and curse that it won't have GPS and that they wish it had and might buy a bluetooth GPS to go with it, etc.

Ok, let's try and set the record straight.

aGPS is always full GPS
aGPS is better then GPS
aGPS is a hardware GPS chip combined with "intelligent" software
aGPS does not require 3G, but is helped by it

There you go!

Some explanation. GPS uses satellites to triangulate it's position. To do this it needs to "see" (direct line of sight is not required) multiple satellites to get a fix. The more satellites it can see, the better. I believe that most GPS chips can see a maximum of 12 to 13 satellites at the same time. Depending on where you are on earth, the maximum satellites you can see ranges from 9 to 15. It is influenced by the curvature of the earth. About 3 or 4 are required to get accurate data. More satellites means better accuracy.

The problem with this system is that the initial fix can take 2 minutes or more. This is because the GPS has no idea where in the world it is, so it needs time to figure this out.

THAT is why aGPS was invented. The "a" in aGPS is software in your device which tries to figure out where you are in the world, quicker then your GPS hardware chip can.

In general it uses 2 methods to do this:
1. It will try to do this using your external internet IP (very rough, but it can pick your country quite easily because of your IP subnet or provider or DNS suffix).
2. It can also leverage your 3G connection to ask the tower it's connected to it's cell tower identification number. Using a database of these towers it will look up the coordinates of that tower and it instantly knows where you approximately are.

It then relays this to your hardware GPS chip so that it can narrow it's search region considerably and give you a quicker fix. Often within 10 to 20 seconds.

Comments

Not sure, but alot of GPS used last position to aid aGPS, the same trick is applied without actually. User experience seems to confirm this with a strong (inverse) correlation between average fix time to current distance to last known position. aGPS is just the addition of externally precalculated localized satellite positioning data.

Something that's being called agps as well right now is the latter option of agps without the gps part. Only the cell tower location information is used then. Happens when the gps module is turned off.

Although a simple matter of misinformation, I've come to learn: 1. the masses are always right. 2. if not, automagically rule 1 applies.

Doesn't mean I like it ofc. With gps module turned on and good reception to the internet true agps can reduce first cold fix time considerably.

I did know that GPS uses it's last know coordinates as it's starting point. aGPS tries to accelerate this even more by using external data. It's sort of the next version of this mechanism you could say. And proven to be more effective, because of as you stated, the distance between the last point your GPS was on and where you are now is unpredictable.

What wheez50 is talking about, is or should not be called aGPS in my opinion. That is just cell tower triangulation. Nothing new under the sun there. Sometimes this will be called "poor man's GPS". But in my opinion labeling this aGPS without a GPS chip in the device is wrong.

hmm, I always asumed aGPS was something else (and WP agrees with me). The fact that a GPS doesn't know where you are applys for any form of geoloaction device. The bigger problem with GPS is that the GPS software also does not know the location of the sattelites since they constantly move around. So until the software knows where the sats are it can't start to calculate your position.

This is where aGPS steps in for the first time. aGPS can use a small downloaded file that contains the location of each sattelite for the upcomuing 7 days. That way it does not need to wait to find the location itself.

The next step is that you need the precise time of the day to make good calculations. When signal strength is poor or the signal needs to bounce too much finding the right time is difficult. aGPS can use other sources to get the right time. without aGPS it's a question of the chicken and the egg. You need time to calculate your location, and you need your location to know the offset of the time signals coming from the sattelites.

It can in some cases use the information from the cell tower to identify it's rough position, but this requires a database for the cell towers or the ability for an online lookup. And not every GPS system has a built in modem. Being online can also help since some systems can offload the inittial calculations to a much more powerfull server.

Using the previous known position is indeed also an option. However if no rough position can be found by assisting services the GPS will try to identify your rough location on the GPS data itself and keep refining it till it has a lock.

GPS requires no more than 3 sattelites to identify your location. A 4th can be used to measure elevation. If you have more sats available it could help a little with more precision but it will be a big help when one of the sats you use dissapears behind a building for example.

The bigger problem with GPS is that the GPS software also does not know the location of the sattelites since they constantly move around

i hope that you understand that your talking crap here? it is US that move around all the time, as the geo-sats stay pretty still. in a geo-stationairy orbit to be precise. - thats actually prettymutch requied since you need to know your location comparered to fixed points (say the equatar, or the north pole)

aGPS lets you download the GPS ephemeris and almanac over your GPRS/UMTS data connection instead of waiting for the GPS satellite to downlink this information to planet earth. It takes you right around 12.5 minutes to get this from the GPS satellite, and over GPRS/UMTS just a few seconds. With this you can get a GPS fix way faster than getting this info from the satellite.

i hope that you understand that your talking crap here? it is US that move around all the time, as the geo-sats stay pretty still. in a geo-stationairy orbit to be precise. - thats actually prettymutch requied since you need to know your location comparered to fixed points (say the equatar, or the north pole)

What Blokker_1999 says about GPS satellites that move around is correct. GPS satellites doesn't follow a geostationairy orbit. They orbit at 20200km. A geostationairy orbit is 35800km. So we move around but the satellites also move.
Source: Computer Networks fifth edition pg. 137-141

@i-chat: GPS satellites are not geostationary!
I agree with quindor that aGPS should not be used as a term to label triangulation using something else than GPS satellites (like cell towers or access points).
I am currently using aGPS in a project and know by experience that this may reduce fix time (from cold start) from half an hour (with no last location data) to half a minute. As an aid to query the assistance server I use the currently connected cell tower (just one!) so I may be off by 35km... in either direction.

i hope that you understand that your talking crap here? it is US that move around all the time, as the geo-sats stay pretty still. in a geo-stationairy orbit to be precise. - thats actually prettymutch requied since you need to know your location comparered to fixed points (say the equatar, or the north pole)

As pointed out by the others, the sats do move around and that is exactly why it takes such a long time to get a fix, take a look at the Wikipedia article about GPS, about 1/3rd down the page you have a nice animated gif showing the movement.

It takes you right around 12.5 minutes to get this from the GPS satellite,

Didn't know it actually takes that long.

RoadRunner, can a cell tower actually broadcast it's position, or do you just get some id number where you have to lookup the location yourself?

in that case i really wonder why they didn't use geo-stationarie sats. isn't that way cheaper than moving sats - even though it may require more.

Nope, geo-stationary is just about where to place the satellites. closer to earth needs more velocity to not get sucked to the eart, further away requires slower velocity in order not to loose the satellite. The problem with geostationary satellites is they all use the same orbit! Hence this orbit is very crowded, plus GPS satellites benefit a larger coverage due to their longer distance to earth.

@Blokker: just their ID, but there are dictionaries in the web with their corresponding locations, this is used for example with Google's location API on cell.